1. Field of the Invention
The present invention relates to a method and an apparatus for thermally recording information in a recording medium and, more particularly, for realizing an excellent recording by controlling the peak temperature of the heating resistors so that it does not exceed a specific temperature.
2. Description of the Prior Art
Conventional apparatuses for recording information in a recording medium thermally utilize a resistor of a metallic compound such as ruthenium oxide or tantalum nitride or a cermet resistor prepared by dispersing an insulator such as silicon oxide into a refractory metal such as tantalum in the heating resistor of the thermal head.
When a proper voltage is applied to the aforementioned heating resistor of the thermal head, an electric current flows through the heating resistor to generate heat energy in the form of Joule heat, and this state is maintained for a constant time to give to the heat-sensitive recording paper the thermal energy necessary for the recording. The energy of the Joule heat generated by the aforementioned heating resistor is determined in dependence upon the resistance of the heating resistor, the applied voltage and the time period for applying the voltage.
The conventional thermal recording apparatus so adjusts the applied voltage or the time period for applying the voltage according to the heat sensitivity of the heat-sensitive papers used, the background temperature around the heating resistor, the temperature of the recording medium itself and the thermal conductivity by which the thermal energy generated by the heating resistor is transmitted from the heating resistor to the heat-sensitive paper that it obtains the optimum recording quality and the desired recording density.
On the other hand, the powered transfer recording apparatus comprises an ink donor sheet having a power heating resistor layer which consists of carbon paint and a power supply head. When the power heating resistor layer is powered by the power supply head, the ink donor sheet is heated by the thermal energy generated by the power heating resistor layer so that the ink may be melted or sublimated and transferred to the recording medium. It so adjusts the applied voltage or the voltage applying time period according to the sheet resistance of the powered heating resistor layer, the temperature of the ink donor sheet and the electrode temperature of the power supply head that it makes the thermal energy generated by the powered heating resistor layer most suitable so as to obtain the optimum recording quality and the desired recording density.
In the thermal recording method of the prior art, for the following reasons, the adjustment of recording thermal energy according to the voltage and the pulse width to be applied to the heating resistor have serious shortcomings which to raise the production cost for the recording apparatus.
The thermal energy to be generated in the heating resistor by applying voltage pulses can be determined in dependence upon the voltage or the pulse width of the applied pulses, as has been described hereinbefore. Despite this fact, however, the temperature of the heating resistor will fluctuate with the pulse applying histories such as the period of applying the pulse and the number of the pulse applied continuously, the thermal histories of the heating resistor, or the temperature of the supporting substrate of the thermal head or the environments.
The thermal recording mechanism depends directly not upon the level of the thermal energy generated by the heating resistor but upon the temperature of the coloring layer of the heat-sensitive recording paper or the ink layer, i.e., the temperature of the heating resistor. If, therefore, it is desired to uniformize the temperature of the heating resistor at the heating time so as to achieve a uniform thermal recording to the heat-sensitive papers or the like, the apparatus needs to collect or predict the thermal data of the environment and histories in which the heating resistor is placed at the instant of heating. It has to so adjust and determine the voltage value or the pulse width of the applied voltage based on those data that the temperature of the heating resistor raises to the desired temperature.
The data collecting means, data predicting means and recording condition deciding means exert seriously high loads upon the hardwares such as a variety of temperature sensors for detecting the temperature of the thermal head substrate of the environment memories for storing the past recorded data so as to account for the recording histories, simulators such as a thermal equivalent circuit for predicting the thermal states, and the CPU and gate circuits for processing data. Seriously complex softwares are also required for supporting those hardwares. Especially, either a large-sized highly precise thermal recording apparatus having a plurality of heating resistors or an apparatus for recording data with a continuous tone of density has to process massive data so that it cannot avoid the increase in size and price while sacrificing the recording quality. On the other hand, the processing time for collecting and predicting the data and deciding the recording conditions is restricted by the CPU or the like and limits the high-speed recording.
Moreover, the thermal head is usually formed with a glazed layer as a heat insulating layer for enhancing the thermal efficiency. This glazed layer is formed by a thick film process so that its thickness disperses over .+-.20% of the average value of the thickness so that the heat insulating effect by the glazed layer randomly disperses among the individual thermal heads. No matter how accurately the data of the thermal environment of the heating resistor might be accumulated and processed to decide the individual recording condition, as has been described herein-before, the highly accurate exothermic temperature control would be blocked by the dispersion of the thermal characteristics of the thermal heads. If a more highly accurate control of the exothermic temperature is to be accomplished, the dispersion of the thermal characteristics of the individual-thermal heads also has to be incorporated as a control parameter so that the mass-productivity has to be seriously sacrificed by adjusting the recording apparatus one by one. If it is necessary to replace the thermal heads in the recording apparatus because of their troubles or lifetimes, it is almost difficult to adjust the settings of the recording apparatus for the individual characteristics of the thermal heads. The dispersions of the thermal capacity and the thermal resistance also depend upon the periphery of the heating resistor layer in the powered thermal recording, thus raising problems similar to those of the afore-mentioned case of the thermal head.